Portable game machines that use dot-matrix liquid crystal display devices for monochrome display are popular entertainment devices. An example of an 8-bit portable liquid crystal display-equipped game machine is the GAMEBOY.RTM. before words which is manufactured and sold by the assignee of this application. GAMEBOY is a registered trademark of Nintendo Co., Ltd. of Kyoto, Japan. The graphics of the GAMEBOY are created from blocks of pixels known as characters. Characters are typically 8 pixels wide and 8 pixels high (or 8 pixels wide and 16 pixels high) and are the smallest addressable unit of screen data for the GAMEBOY. Each pixel can be one of four shades of gray. Characters can be used to represent both background and object data. Backgrounds are groups of characters laid out like tiles to create the environment or background of a game. For backgrounds, a portion of memory corresponding to the screen is used to point to character data to be displayed at a given location. Objects are groups of character data that are used to represent the main game character and other moving objects that overlay the background in a game. Objects are sometimes referred to as "sprites". Objects also occupy a range of memory pointing to character data, but in addition they have other attributes such as screen location and priority. Thus, characters are displayed in monochrome tones on the screen of the liquid crystal display.
Japanese Patent Application Laid-Open No. HEI 7-204349 and counterpart U.S. Pat. No. 5,556,108 (the contents of which are incorporated herein) disclose a system in which a conversion device permits the use of a cartridge for one type of game machine with other different types of game machines. For example, the conversion device enables the display of originally monochrome images as color images. In this case, the conversion device is used with a game cartridge for a first type of game machine that displays monochrome images to generate image data for a second type of game machine capable of color display. This conventional technology makes it possible to display color images by providing the conversion device for color conversion processing between the cartridge and the second type of game machine.
However, while representing an improvement which permits the use of game cartridges with different type of game systems, the system described in the '108 patent requires that the conversion device be connected between the second type of game machine and the cartridge in order to color-display monochrome images. Moreover, background images and object images are color-converted by using only one color palette, so that the displayable colors are considerably limited.